Cirque. A deep, steep-walled recess in a mountain, caused by glacial erosion at the head of a valley. Concretion. A nodular or irregular concentration of minerals such as calcite or limonite, formed by precipitation of the mineral from groundwater around a nucleus. Conglomerate. A rock containing coarse fragments of an older rock, usually as rounded water-worn stones or pebbles. Conodont. One of a group of tiny dark brown tooth-like fossils thought to be dermal or dental parts of some extinct group of fish. Diatreme. A volcanic vent or pipe drilled through rocks by the explosive energy of gas-charged molten rock, now containing igneous rock and often altered or unaltered fragments of the surrounding rock. Dike. A vertical or nearly vertical sheet of igneous rock which cuts across the structure of adjacent rocks. Diorite. An intrusive igneous rock composed of sodium-rich feldspar and dark minerals, with only small amounts of quartz. Dip. The angle at which a layer of rock is inclined below the horizontal. Dome. A roughly circular upfold in which the rock layers dip outward in all directions from the center. Dowsing. Searching for underground water or ore with a divining rod, usually a forked stick supposed to locate spots where the desired substance may be found under the surface. Echinoderm. One of a large group of marine invertebrate animals, most of which have pentagonal symmetry and a skeleton of many calcite plates. Many forms are spiny. The group includes starfish and sea urchins. Evaporite. Chemical sediments precipitated when water (usually sea water) evaporates. Extrusive rocks. Igneous rocks formed when molten rock material is ejected onto the surface. Synonymous with volcanic rocks. Fault. A break in the rocks in which there has been displacement of the two sides relative to each other. Fault block range. A mountain range bounded on two or more sides by faults. Feldspar. A group of light-colored aluminum silicate minerals that are major constituents of igneous rocks. They contain potassium, sodium, and calcium in differing proportions. Fold. A bend in rock layers. Foraminiferida. One-celled marine animals with microscopic, perforated, many-chambered calcium carbonate shells, often called forams. Fossil. The remains or traces of an animal or plant which has been preserved in the rock. Fusulinid. One-celled marine animals (forams) with shells which look like a grain of wheat in shape and size, frequently abundant in Colorado Pennsylvanian rocks. Galena. A heavy gray metallic mineral (PbS), often cubic in form, that is the most important ore of lead. Gangue. Nonvaluable minerals occurring in veins with ore minerals. Glaciation. Alteration of the earth’s surface by erosion and deposition by glacier ice.